Depends upon what you mean.
Since T20 is D20, all creatures created for the D20 system can, in theory, be imported to T20 with (almost) no changes.
In the last playtest draft there were the create Animal Encounter table system, adapted from CT Book 3 to generate D20 creatures. So if you could roll the creature up on those tables, you can call it an alien race and use it directly.
If I can interpret, the real question you are asking is "What are the guidelines for fitting [Creature x] into the Traveller universe?". Which is a harder question to answer clearly.
* If the creature requires magic, or uses magic, it won't fit.
* If the creature requires or uses psionics, it won't fit.
* You need to think more about the envornment in which the creature lives, what it eats, and how it reproduces. For intelligent creatures, be very aware of their social interaction. Loner creatures do not build great civilizations. Xenophobic creatures tend to be isolated. Psychotic races would either kill each other off or get themselves killed off.
My experience with creating alien races for Traveller, and watching other do so, that there is a huge range of possiblities, but the Traveller universe "Hard Space Opera" places some (apparently random) restrictions on what can be done.
The last point would be: Try to maintain the canon distribution of alien races. There should be about one Minor Human Race and two or three alien races per sector. Both the Spinward Marches and Ley sector (at last count) were over this limit, so you may want to try and spread your's thinner.
If you don't mind constructive criticism, post a few general ideas and we'll offer some suggestions.