M8 thread has a depth of .76mm so you would need to turn it down to about 6.5mm to get back to round bar. The pitch is different as well so you wouldn't be able to use the remaining M8 thread if it was turned down to 7mm.
yeah you can get M7 x1.0mm thread. if the M7 thread needs to be finger tight you may be ok as you'll have a partial thread, but if it needs any strength then no, so it'll probably be an no.