No, they won't work.
They look the same, as they use the same hardware, however, the Partner system (the phone system) uses different inband codes to communicate with the voice mail system. You can not change the inband codes on the voice mail, they are hardcoded into the software of the voice mail.
Now if you plug in a Partner Mail to a Legend, you may get the voice mail to answer, but you won't be able to have auto attendant or direct transfer to voice mail. Infact the voice mail may simply pick up and hangup after a second, as it won't understand the DTMF's the Legend is sending to it.
Sorry, it would be neat if you could do this.
Neil