Putting aside the very significant issue of competence.
You shouldn't. It it very important, but don't imply FP's can't be competent. Not through a weekend course, but it is possible.
Where is the OP going to recruit patients to feed your (not mine, endo is my passion, not echo) echo machine ? Unless they are very rural I don't see this working out.
It depends. I did some locums work for a large practice (~25 FP's) just after residency in a city of ~700K. One of them read ECHO's. They had a huge patient base and also did CT's, MRI's, mammo's, etc.
I work in a rural area, it's still not easy to recruit patients. I pulled a buch of chicken bones out of uninsured patients at three in the morning before the other guys in town started sending patients my way.
As I indicated previously the machine is not cheap, and you need a tech.
Buying a machine isn't the only option, there are lease agreements, shared ownership, and other ways to decrease overhead.
This just doesn't sound like a good idea at all to me.
It probably isn't a good idea for you, but it's not a bad idea for everyone.
If you want to make extra money, do ER work, Botox, insurance / IMEs, pilot physicals, etc.
All of these things can get you "into hot water," too.
There are so many other ways to generate extra revenue that won't get you into hot water.
Do it if you love it, not for extra income.