How high up on the creepy scale is adding on Facebook, a saleswoman you spoke to in a shop?
1 being this isn't in anyway inappropriate, i'm so glad youstalkedfound me! And 10 being omg you psychopath i'm calling the police!
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That is definitely into the creepy end of the scale... (especially just randomly adding them with no message sent).
And going back to the store specifically to see her could also be perceived as creepy. Unless I could pull off the nonchalant "Oh hi I'm glad I bumped into you again!" move.

Why lie about it? If you want to ask someone out then just be honest and ask them out, why the need to pretend? It is just going to be weirder if you make some half arsed/confused attempt at getting to meet her, just make it clear you like her and would like to take her out.
If she does accept your friend request then please do just message her and be upfront about it, just tell her you like her and ask her out.