As a user, when I press Ctrl + Enter (Windows shortcut) on a text block without checkbox, I'd like for a check-box to be created

There's an implication here. If such a thing were implemented AND given that enacting such a function should allow the "state" of a text block to cycle back to default settings, then the new state change as a result of enacting this operation on a page will be as follows:

• Normal text block, b

Use command

• Block b is checkbox

Use command

• Block b is "checked off" checkbox

Use command

• Block b is normal text block (back to state 1)

I might've overcomplicated this. Just wanted to share my thoughts in a clear way.