Originally, Rolemaster was written as an addon to the original D&D.
Here is a way that you could easily add RM criticals to d20 (D&D 3.5). Basically, if, in 3.5, you roll a critical, you roll on the critical table instead of just doubling damage. This is basically an off-the-cuff method.
If a weapon has a crit range of 18-20, then it will do A (18), B (19), or C(20) criticals, and so forth. Other weapons would be based on their crit threat ranges.
Roll, add your BAB, if the dice roll is in the crit threat range, and the attack hits, then roll on the critical table accordingly...