How to add a new item into the end of an existing line?

[Keith Clarke]

Folks,
Please can anyone clarify the syntax needed to add a new item A to the end of an existing line B of an existing variable C?

I can add to the end of the variable OK, as in

put foo after C

but if I try to put foo after B, after line B or after item -1 of line B, after Z (defined as item -1 of B) etc., and I get either no result or compilation errors.

I’m clearly missing the magic words.  

Thanks in advance
Keith..