You would need to use policy nat/static nat in order to differentiate when traffic will use that IP address only when going to the tunnel, something like
access-list VPN permit ip
static (inside,outside) Y.Y.Y.Y acces-list VPN
and the crypto map will use that Y.Y.Y.Y as the source of the vpn traffic.
Now one little catch here, if you are going to use a single ip address, then PAT is required and the config will change, causing this not being bidirectional (only replies to traffic from your inside network will come back, not traffic originated from the remote network) for PAT use
access-list VPN permit ip
nat (inside) X access-list VPN
global (outside) X Y.Y.Y.Y
hth
Ivan
