It's possible, for instance Joseph Rezeau has done it to include his regexp questiontype into the cloze question type
Unfortunately cloze code is not structured to allow this easily so you will need to change several places in the code.
And of course the main drawback of doing this is that each Moodle upgrade will be a pain.
Maybe a better solution would be to modify the code of the algebra question type so that it can be used with the combined question type . It's quite easy to modify a question type so that it is "combinable".
I must admit that I never had the idea that algebra could be used as a subquestion so I never looked at what could be done in this area.