Yes you could absolutely do that. First, do you really mean .6 and 1.2 milligrams (per milliliter, that is, even though it doesn't always say so), or do you perhaps mean .6 and 1.2 percent? They differ by an order of magnitude, so 1.2% is 12mg/ml, and .6% is 6mg/ml. I ask because the figures you gave, .6 and 1.2 milligrams, reflect an extremely tiny amount to use and would essentially be like using no nicotine at all. If that's what you mean then great, I just want to make sure you don't think you're getting one thing and end up with something else instead.
I don't know any particular vendors you could use since I'm in the US, but I know there are EU sources for DIY nicotine, because I know there are EU rules for how strong it can be. 72mg/ml (7.2% by volume) is the limit, and you'll probably want to buy it in that highest strength available, as it's the most cost-effective.
To determine the amount of nic base to add to your premixed liquid requires a little bit of math. There are tools out there than will do all this for you, but I don't recommend mixing e-juice if you don't know why you are using the amounts you are using. So let's do it the long way, this time. (Also, I'll presume you meant 6 and 12mg, not .6 and 1.2 -- if that's wrong, you can use this same method with those numbers to achieve the correct result).
So for starters, you get a 100ml bottle, at 6mg/ml that's 600mg total nicotine in the bottle.
You want to add "x-amount" in ml's, at a strength of 72mg/ml, or 72x in mg. Added to the initial 600, that's a total of 600+72x (in mg) in the bottle.
And when you're done, you want it such that the resulting liquid (100ml + x) has a strength of 12mg/ml. That means it will have a total of 12 * (100 + x) mg's of nicotine.
So now, we can make an equation. We know the amount of nicotine in the bottle after we add some, we put that on the left. We know the amount of nicotine we want in the bottle when we're done, we put that on the right. And, we want these two (the amount we have when we're done, and the amount we want when we're done) to be the same thing, and so we put an equals in the middle. All said and done, that looks like this:
600 + 72x = 12 * (100+x)
600 + 72x = 1200 + 12x
600 - 600 + 72x = 1200 - 600 + 12x
0 + 72x = 600 + 12x
72x = 600 + 12x
72x - 12x = 600 + 12x - 12x
60x = 600 + 0
60x = 600
60x / 60 = 600 / 60
x = 10
So you'll want to add 10 (ml) of 72mg nicotine. Great, we're done here, right? No, let's check our work first:
10ml at 72mg/ml adds 720mg to the bottle, which already contained 600mg. 600 + 720 = 1320mg total nicotine in the bottle, after mixing.
10ml added to 100ml gives 110ml total volume, after mixing.
1320mg / 110ml = 12mg/ml, which is what we wanted, so now we can be done. Handy that it's a nice round number like that.
It gets easier once you've done it a few times.
