How to add more to a blend?

When I create a blend manually, I seem to be limited to 10 dimensions and 10 metrics. But if I create a blend by blending two tables, it seems that I can add as many dimensions and metrics as I want to the tables and the blend can accommodate them. Is there a way that I can add more dimensions and metrics manually once I've done this? I have a blend all set up (created by blending two tables) but I need a few more fields. I would prefer to add them manually rather than going and setting up the two tables and reblending. Any hints welcome.

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