This development lets you "act as if you have an extra White and Yellow explorer".
Can I then reassign these virtual dice, or are they just fixed as extra explorers?
From page 6 of the manual:
Most developments grant a power once built, usable either during the Assign step, or in the phase(s) shown.
Alien Research Ship indicates that its power is usable only during the Explore phase, so you cannot reassign those dice, because dice cannot be reassigned during the Explore phase.