The 5324 will have to call the 3rd party and add them in.
Else....
If you absolutely need to be able to accept a call from a 3rd person.
Add a second line, forward on a busy to it. Then when the call comes in, you answer, press xfer conf button, dial the remote hold pickup code of the original line, then hit the conf button again.
Easier just to initiate the call.
Ralph