Hi I have been working on this sticky problem for a long time, I had come up with some general rules of thumb for conversion, some of it provides guidelines for flavor, some were mechanics
A.The basis for feel in any game might be the description of difficulties
In Traveller you have checks and descriptive difficulty
2 Simple
4 Easy
6 Routine
8 Average
10 Difficult
12 Very Difficult
14 Formidible
Similarly in d20 you have DC's and a descriptive difficulty
0 Very Easy
5 Easy
10 Average
15 Tough
20 Challenging
25 Formidable
30 Heroic
35 Superheroic
40 Nearly Impossible
Hmm.....
Check Traveller = DC D20
2 Simple = 0 Very Easy
4 Easy = 5 Easy
6 Routine = 8 (?)
8 Average = 10 Average
10 Difficult = 15 Tough
12 Very Difficult = 20 Challenging
14 Formidible = 25 Formidable
16 = 30 Heroic
18 = 35 Superheroic
20 = 40 Nearly Impossible
Not perfect, but why not....
B.Stats
Well, an unmodified human has
In Traveller, a range of 2-12 centered on 7 with an unmodified natural maximum of 15
In D20, a range of 3-18 centered on 10 with an unmodified natural maximum of 23 (those +5 points for 20 levels). At least if you are not doing epic stuff
So it seems that at least
Traveller = D20
2 = 3
7 = 10
12 = 18
15 = 23
or something like that.
Now as to what attribute converts to what. STR, DEX, CON are pretty easy. Because of its influence on psionic resistance checks in other areas in Traveller then it use as Will saves in D20, I suggest using D20 Wisdom as Traveller Intelligence. Because of it use in skill ranks, D20 Intelligence is Traveller Education. This is not perfect as D20 Intelligence directly relates to animals vs sentients, but as was said above, conversions are not perfect. Also, I still have not figured out what to do with D20 Charisma does not relate to Social Standing and Humans are not Vargr (least not the ones I know).
C.Skill ranks
Because of the assumptions in part A and B, Traveller places a different emphasis on the relative importance of a skill level vs characteristic DM in its checks as opposed to D20 ranks in a skill vs attribute bonus in a DC. Seems to me that if you separate the total D20 bonus into skill ranks and attribute bonus, you can start doing conversions.
Now that you have your skill ranks, in D20 have at most 23 ranks by 20 level.
Hmm....
Having a single skill rank somewhat like having a skill level of 0. Having 23 ranks may rate you at a skill level of 4 or even higher. Divide by 4 or 5 (round up or down) or some other number, and you have something like a conversion of you may need. I found dividing by 5 and rounding down, you get a pretty good range ranks to skill levels. Even works for Base Attack Bonus, when you think about it.