SNAT in Transpernt mode

Hello Friends

We currently have fortigate cluster 500E in A-A mode

i have divided the firewall to vdoms ,and made a transpern t vdom in which the WAN port is assigned

all vdoms have a vlink with a REAL IP address on the vdom link side

my need is to do SNAT outside for a UC device with another ip from the pool

how can achive this ?

thanks !!!!