asterisk users - Mar 2008 - transfer call

Hi there,

i'm beginner in Asterisk.. I have situation:

when a caller ,some SIP account on local network, dial outside number
trough some Zap channel, asterisk have to automaticly transfer call to
another local SIP channel....In other words, he have to change
caller..

For example:

I have SIP accounts 100 and 101..

WHen i call some outside number , PBX must to automatic switch the
caller...like from some softphone , when i press transfer...
Is it posible trough asterisk , extensions.conf etc...