Font Creator offers MANY tools to produce the results you want.
In addition to those already mentioned (Composites, cut and paste) is using the “Add Rectangle” and “Add Ellipse” Tools at the top of the page. They may be even faster than cut and paste.
Another is Samples to change the characters shown on the left side of the screen that can then be drug into any editing window (IF they are not composites in the original font):
