Assuming that it is a newer ACS (not sure about anything before ACS R3) configured as a key system (not pooled/hybrid):
The cordless user can perform a hook flash to place the call on hold. Any other phone that can access that line can then grab the call. Just don't hang up the cordless until someone else takes the call or it will ring back to the cordless.

I just checked the manual for ACS R7 and it says this:
Using Hold on a Single-Line Telephone

Follow these instructions to place and retrieve a call on hold on a system telephone:
To place a call on hold, press the switchhook down once rapidly.

You hear intercom dial tone. The call is on hold. (Do not hang up the handset while the call is on hold. If you hang up the handset, the telephone rings. Lifting the handset reconnects you to the held call.)

To retrieve a call that you put on hold:
1. Hang up.
The call “rings back.”
2. Lift the handset.
You are reconnected to the held call.

I hope that helps.