Combining data from 2 spreadsheets

At my workplace the salaries are usually calculated in a simple Calc sheet
that has 3 columns; the employee ID, his name and the salary amount. The
number of paid employees ranges, month to month, between 40 to 100.
We now need to add a bank account number to this list. The account numbers
are in a separate spreadsheet that has 2 columns employee ID and his
account number. This second spreadsheet has over 1500 entries in it. Is
there a way to automate filling the account column in the first sheet from
the second by using the employee IDs. Thank you.

Regards,

Try adding a 4th colum in your sheet with salary's with this formula:
=VLOOKUP(A2;'file:///C:/Accountnrs.ods'#$Sheet1.$A$1:$B$15;2)

Of course you should change the reference to the correct sheet,
and if thing are unclear read the help here:
https://help.libreoffice.org/Calc/Spreadsheet_Functions#VLOOKUP