Hi goudacheese,
Yes, mixing the mould into the milk does the same thing. Basically, it gets the spores distributed throughout the milk so that they will grow later. Blue requires oxygen to grow, which is why you want lots of openings throughout a blue cheese and why you pierce them.
If you harvest the mould from a cheese you like, just be very sure that you've sterilised everything (like the spoon you use to remove the mould, the fork you use to mash it up, etc). You can harvest white mould from camembert as well.
As pointed out, you have less control over what you're adding and by doing it this way you do increase the risk of contamination. However, if you keep things clean and are careful, it should go well.
- Jeff