Assuming the data you have imported is in Sheet2, and you have the
unique usernames in column A with the numbers you need in column D (for
example) and there are 100 rows of employee details, and that in your
worksheet (Sheet1) you have a list of your employees' usernames in
column A with headers in row 1, then in cell B2 of Sheet1 you can enter
this formula:

=VLOOKUP(A2,Sheet2!A$2:D$101,4,0)

This will try to find an exact match between A2 in your sheet and
column A of Sheet2 - if there is an exact match then it will return the
corresponding value from the 4th column (i.e. column D) of Sheet2. The
formula can be copied down for as many employees as you have in column
A.

Hope this helps.

Pete