If you want to have both systems (current alarm panel/keypads + SmartThings) active at once, going with a solution like presented earlier in this thread may be the best solution.
If you don’t want to keep using your existing alarm panel and keypads, then going with @heythisisnate’s solution above is a very good, popular, low cost solution.
If you’re more comfortable in the Arduino IDE you could use my ST_Anything solution. I have an example sketch for the NodeMCU ESP8266 which is designed for use as an alarm panel replacement. This solution is very similar to Nate’s, but uses the Arduino IDE instead of LUA. My solution also does not require the use of an OAUTH SmartApp. There are pros and cons to all of these solutions. The nice thing about Nate’s solution and mine ST_Anything is they both run on identical NodeMCU ESP8266 hardware and can be wired exactly the same as well. Just the software on the ESP8266 and the ST Cloud are different. For < $25 bucks, you don’t have much to lose.