I’m curious if there’s a way to add an alarm or make smartthings work as an alarm. I have door sensors and motion sensors, so I’m hoping to not have to duplicate those sensors for alarm functionality. I don’t want my doors covered with different contact sensors, if that makes sense.
I’m sorry, I don’t understand this sentence.
If you are just asking if there are sirens that work with SmartThings, sure. See the official compatibility list. They are in the “other” category.
That said, SmartThings itself is just not reliable enough to be a primary security system. That’s not just my opinion, the company says so:
https://www.smartthings.com/guidelines
Data accuracy and consistency from SmartThings sensors, including those provided by SmartThings directly, resold by SmartThings, or supported by SmartThings, is not guaranteed. Therefore, you should not rely on that data for any use that impacts health, safety, security, property or financial interests.
If you do add a siren, locate it somewhere where it will be easy to turn off manually, and be prepared for occasional false alarms. 
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I meant doors covered with different sensors, but based on what you’ve shared, a stand alone system sounds like a more reliable option. I’m not sure if a z wave contact switch will be more or less reliable based on the hub it uses though, I guess I don’t see how a wireless dedicated alarm is inherently more reliable, but I’ll keep researching. Thank you for the information.
Yes you can reuse your sensors for multiple purposes in SmartThings. They can serve as sensor to trigger automations and also security apps and anything else. As JD pointed out you can buy an alarm or siren, pair with SmartThings and then use your sensors with the built in SHM security module from SmartThings or you can use any custom apps such this one to create your own security setup and trigger the alarm for an intruder alert or do much more.
Any system is only as reliable as its weakest link. You in this case you’ll have to ensure that your hub and internet are on a UPS for it to be reasonably reliable.
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A system which is purpose built to be an alarm system will almost certainly be more reliable than smartthings. For one thing, smartthings is mostly a cloud-based system, for example all of the notifications come from the cloud. Most purpose built alarm systems, even if they use zwave, will operate everything locally or both locally and the cloud rather than relying just on the cloud. The better systems will have cellular communications built in.
But there are other issues specific to smartthings as well.
If you want an inexpensive security system and you are in the US, Ring Alarm is worth looking at. But there are lots of choices. 
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That all makes sense, even the best secure, has the same disclaimer as smartthings about security and safety, I’m not sure any alarm company won’t have a waiver in place to protect themselves from litigation in the event of equipment failure. I’ll explore that suggested app, and the monitoring is just a layer if security, more applicable to an empty home than an occupied one. There are other factors at play to keep the home safe while occupied.
You can buy an alarm
For example the Aeotec Siren thar i use and pair it with smartthings monitor for offline security
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I’ll read up on ring, after reading the terms of simplisafe and nest they don’t veer too far from the smartthings disclaimer, and they are dedicated alarms. There’s so much information in the smart home space, it might take a bit, but to parse all this information, but it’s all appreciated.
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