AFAIK you can't do this with anything simple
The best way to do it would be something like...
"appvTemp"⇒Str1
"appvName"⇒Str2
GetCalc(Str1, new size)⇒A
GetCalc(Str2)⇒B
Copy(B,A,old size)
<data>⇒GDB1
Copy(GDB1,A+oldsize, Δsize)
DelVar Str2
GetCalc(Str2, new size)⇒B
Copy(A,B, new size)
DelVar Str1
where appvTemp is a temporary appvar, appvName is whatever appvar you're using and <data> is whatever data you want to add
if there's any easier way to do this, I'd be interested as well