If you can actually develope you can simply Create a Gui that does that.
Basicly you need to:
Read Charnames from ps_game
get CUser instance of the User you want to PM to
Send PM Packet to the User
If i remember correctly, CUPS released examples of
all of this somewhere in the Forum except for creating the
Gui and Combining the Steps i guess.
Also through some Forum posts, you can Find enough Informations
about the PM Packet.
ps:
ive never looked much into the Serverside of Shaiya,
( except for some Packet related / security related stuff )
so there might be other Methods .
But the one i told you is fairly easy to make and easy
to use for Admins.