With Assignment Group Weighting, you can have one overall scheme. For example:

Project A - 20%

Project B - 20%

Project C - 20%

Final Exam - 40%

Once you have set up the overall scheme, that's it. You can't add an additional level. You can't create a subset of Assignment Groups for only Project A.

That being said, you can work with the point values. For example, let's say that Project A is worth 100 points in total. Then just create separate assignments that add up to 100 points. For example:

Proposal - 10 points

Sources - 20 points

First draft - 20 points

Final draft - 50 points

Here, with points, I've made the final draft of Project A worth 50% of the overall Project A grade.