I think you could do this, but I don't know if you should do this (although I'm happy to see you're including a preservative!). A big part of using the 0.5% in the cool down phase of our products has to do not only with the extracts not really liking heat, but also because of the solubility of the powders and figuring out how much you're using in a product.
You can see this with the toners to the left. Number 2 has too much dissolved in it, and the extracts eventually sunk to the bottom and formed a gooey precipitate!
Let's say pineapple extract has a solubility of 0.5 grams in 100 ml of water at 25˚C*. You dissolve 0.5 grams into water, glycerin, and preservative (total 99.5 grams of solvent) and have a lovely botanical extract system. You can't add more than 0.5 grams to this 99.5 gram bottle of liquid because the extract will precipitate. So what we have is a bottle with 0.5 grams of pineapple extract in it.
*I can't find the solubility of pineapple extract so this is just an example amount.
If you wanted to use 0.5% pineapple extract in a product, you'd have to use the entire bottle to get that amount. If you wanted to use less, you'd have to do the math on it (0.25 grams in the product would mean I have to use 50 grams from the bottle, 0.125 grams in the product means 25 grams from the bottle, and so on). This simply isn't doable because you'd need to add a ton of the liquid to get a small amount of the extract.
If we did this with an ingredient that had a higher solubility than our pretend amount for pineapple extract, this might be doable. For instance, let's pretend green tea extract has a solubility of 5 grams in 100 ml of water. Weigh out 5 grams of green tea extract, 0.5 grams of liquid Germall Plus, and 94.5 grams of water, and you've got yourself a liquid green tea extract. How do you figure out how to use it in your products? We can guess that using 10% of this liquid would give us 0.5 grams of green tea extract in our products. It's not completely accurate, but good enough.
If you preserve it well, you could make a liquid extract, but it will always depend upon the solubility of that ingredient in water, so consider using alcohol as your solvent. An ingredient with low water solubility could have a higher alcohol solubility, and that might be a better choice for your solvent. For instance, salicylic acid is soluble at 0.2 grams in 100grams of water but about 25.4 grams in 100 grams of ethanol (my math might be off here, but you get the general idea), which means alcohol is probably the better solvent in this situation...but then you have to ask yourself if you want alcohol in your product!
I don't know much about the topic of making tinctures, but here is a link at at New Directions Aromatics that might be helpful.
There are good reasons for using a liquid extract. I like to use my liquid green tea extract in lotions when I don't have to worry about dissolving it in the cool down phase and I don't have to worry about having a browny-greeny coloured lotion, but I wouldn't get away from the colour and the heat sensitivity if I made my own.
I think it's do-able, but I don't think it's necessarily wise and I don't think you're going to get what you want if you do make it (although I love to be proved wrong!!!). The extracts you're buying from the suppliers aren't powdered extracts dissolved in water - they're manufactured to be mostly colourless liquid extracts that will keep for quite some time on your shelves.