On my host, I can edit .htaccess inside MODX, but it shows as locked in the MODX file tree and I get an error message when I try to save it -- though it is, in fact, saved.
This is a bad practice, though, (as you found out) because one tiny error in .htaccess and you're locked out of the Manager. It's much better to do it outside of MODX, back up .htaccess before each change, and check to see if the home page loads in another browser after each change is saved.
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PLEASE, PLEASE specify the version of MODX you are using . . . PLEASE!
MODx info for everyone:
http://bobsguides.com/MODx.html