Sorry I completely missed the point of your post first time I read it.
There are two ways you can add a string onto the end of a var, the first method will prevent duplicates, the second won't.
Method 1: set %var $addtok(%var,<new data>,32)
Method 2: set %var %var <new data>
Using method 1 the last parameter of $addtok is the ascii number of the separator you want to use, which in this case, is a space (32).