If by "write off" you mean to move part of the balance owed off the loan and to your loan loss allowance GL, then yes.  In CU*BASE we refer to that not as a write-off, but as a partial charge-off.  (See the related link below to understand the important difference between these two terms on CU*BASE.)

You can manually do a partial charge-offs on a loan by using Tool #492 Member Account Adjustment (Full) to post an adjustment to the account and move the desired amount to your loan loss GL.   

But be careful: A charge-off handled this way will NOT write any data to the CU*BASE loan write-off/charge-off database, and you will have no historical data as to the reason for the charge off or why the loan balance changed. See the links below to learn more about how this database works.