I know you know this, but just to clear up any confusion…
Both Zwave and Zigbee use the term “association“ but they use it in different ways.
ZWAVE ASSOCIATION: ONE TRIGGER DEVICE TO ONE OR MORE TARGET DEVICES
In Zwave, you set up an “association group“ so that a trigger device can send a message to one or more target devices without having to go through the hub. You can currently set these up in smartthings by using the Z wave Tweaker tool that was developed by the community.
Once the commands are sent, they look like any other network commands. The trigger device is the sender and the target device is the destination address.
The trigger device has to get permission from the hub to be able to do this, and it specifies the specific targets it wants to be able to talk to. So that is called setting up an association.
Note that one Z wave device might be in different groups for different trigger devices. So the hall light switch might be in association group 2 for an auxiliary light switch at the other end of the hall and then association group 3 for a handheld remote. Or it might be in two different association group 2’s used for two different trigger devices.
The point is that the zwave association group is specific to each trigger device. It’s just the list of device IDs that that trigger device is allowed to send messages to.
ZIGBEE ASSOCIATION: ANOTHER WORD FOR JOIN
In Zigbee, the term “association“ refers to joining the network for the first time. The new device gets associated to a parent device. But that doesn’t have anything to do with groups.
ZIGBEE GROUPS: GROUPCASTING
In Zigbee, groups are used for “groupcast“ commands. Once the device is in a group, it’s in that group no matter who the trigger device is. A trigger device which is capable of groupcasting Can then send a a single command to group 1 (instead of to individual device IDs) and every device on the network will check to see if it is in group one and if it is, it will then perform the requested action. So this is a very different mechanism than zwave direct association.
At the present time there is no way in a smartthings set up that I know of to set or change the Zigbee groups that individual devices belong to.
ZIGBEE END DEVICE BIND
Zigbee has another function called “end device bind“ that you can do in smartthings with custom code which allows a Zigbee trigger device to send a message directly to one specific endpoint on one specific Zigbee target device. So there’s no group involved. But it is similar to zwave direct association if you were only looking at two devices, the trigger and the target.
Here’s an old discussion of how to do that in smartthings, which I believe is still applicable for now. But I don’t know if you’ll be able to do it once the new platform is fully in place and the groovy IDE goes away. They have yet to publish the details on hub connected devices and custom code.
SUMMARY
I haven’t looked at the specs, but it sounds like the Frient Alarms are depending on groupcasting, and as you noted that is not currently supported in smartthings.
You might be able to individually bind one frient alarm to another, again I haven’t checked the specs, but it would be a lot of work. 
And Again, I know you know all of that, but I wanted to spell out the details for people who are less familiar with the protocols. Just to avoid any confusion. 