it wont add to the stickers but you can still put it in your memes if you search it in the search bar to add transparent imags, it will say in the bottom ''my images'' and you pick the transparent one u want
Let's say you have a good transparent transparent image on your computer or tablet: you can simply upload it as a standalone template and it will remain a transparent sticker once it's on imgflip.
You need to use relatively high quality images in the right png format or they won't remain transparent after being uploaded.
I'm using free transparent images sites like "kindpng" to find the stickers I need.