Yes, it's possible to have multiple XBee's sending to a single other XBee ( although I admit I haven't tried it ).
I believe that should "work out of the box" providing you aren't sending huge amounts of data or with prolonged gaps in what you do send, but you may have to tweak some settings if not.
To make it work - All XBee's must be on the same Channel, have the same Pan ID number, and whatever the "MY" destination address is for the receiver must be set as the 16-bit short nickname in "DL" destination address. This can all be setup from the Programming Editor XBee setup Wizard.
Get a one-to-one working first, then get the last working one-to-one, fnally check two-to-one also works.