How To Get A Macro To Search Another Workbook

I have a simple spread sheet that I made that takes the first letter of a name entered into cell (A1), combines it with the last name entered into cell (B1), and inputs that combined information into cell (E1).

It works how I need it to for the most part but... I need Excel to look at a completely separate workbook (containing four sheets), and search the entire workbook for exact matches of the combined information in (E1). If it finds a match elsewhere in that workbook I want it to add a 1 to the end of the combined name. I basically want it to increment the number until it does not find one that already exists in the other workbook.

Example: Cell (A1 contains Bob), Cell (B1 contains Smith). Currently Cell (E1 would display bsmith) - I need for Excel to search the other workbook, and if it finds a bsmith anywhere in that workbook add a 1 to the end making it (bsmith1). If it finds (bsmith1) increment it to (bsmith2) and keep going up by one number until it does not find any other matches. The other workbook is located on a mappedshared network drive.