The call transfer is available as a premium feature in Zoiper. It i as a part of the XFER, CONF, REC, WB, ENCRYPT pakage. Alternatively there is also a Zoiper Premium that can be seen from the next link, which has this feature.
Keep in mind though, that if the PBX does not support call transfers or they are not allowed for your account, you will not be able to use them. Check with your provider for more details about that.