You can go the way Kristin suggests,but you cannot default this the way the OTTB manager fields work. (they would need to go into the lookup and select).
with one simple update statement in all investment objects it is possible to utilise the current user macro.
i.e. setup the lookup like normal with userid as the hidden id. Run update statement for that lookup. magical current user option then appears.
Note: does not work for custom objects without some real hacking
If you want to know get in touch, obviously use at your own risk.
(sorry not posting customisations in the public forum!)