i think i found a similar scenario for what i'm facing in the following documents
NAT Source Addresses through VPN.pdf
ScreenOS_VPN_with_Overlapping_Subnets.pdf
i'm gonna try this one, but it will be great if anyone can confirm that these documents are going to solve it.
http://kb.juniper.net/kb/documents/public/VPN/ScreenOS_VPN_with_Overlapping_Subnets.pdf
http://kb.juniper.net/kb/documents/public/ApplicationNotes/Technical/ScreenOS%205.0.0/NAT%20Source%20Addresses%20through%20VPN.pdf