If the names are in column A, then you can use a formula like this in
a helper column:
="xy"&A1
and copy this down as required. Then you can fix the values in the
helper column by highlighting all the cells with the formula in,
clicking <copy>, then Edit | Paste Special | Values (check) | OK then
<Esc>. Then you can copy/paste these over the originals in column A
and then delete the helper column.