You can use string split for this,. I've used a semi-colon, and $local_joinedList$ starts off empty.
If (certain condition is met)
Variable Operation: $local_joinedList$;$local_newValue$ To $local_joinedList$
End If
String Operation: Split "$local_joinedList$" with delimiter ";" and assign output to $my-list-variable$
This overwrites $my-list-variable$.
If you need to append to an existing list, you can do it the same way by using String Join first, append your values to the string, then split it again afterward.
String Operation:
Join elements of "$my-list-variable$" by delimiter ";" and assign output to $local_joinedList$