Can i add GPU onboard to a laptop with only CPU (integrated GPU) ?
99.9% of the time, the answer is no. To know if you have the 0.1%, you'd have to provide better details.
The best bet would be a laptop that was high-end enough to use an MXM slot, but it was the lowest laptop in that high-end line and did not come with a discrete GPU.
Some random budget laptop, the answer will be bordering on 100% no and the only reason I'm not saying 100% is that I'm never
that certain on anything and there can always be some obscure, odd exception.