I haven't tried this myself yet but I suppose you can change the existing bank transfer plugin if you're not gonna use that and change it to fit your needs.
Basically you would just change the name of the plugin to "Pick up" like you said, or something similar and then in the description put a small explanation like "Choose this option to pick up your order and pay in the store".
Then in the 'Bank account information' field you would put something like "Thank you for your order. Your order will be prepared and you can come to pick it up and pay in the store" and then maybe some address information and store opening hours.
It should work.
Of course, if you also want to use the bank transfer plugin you would have to copy that first. I see there isn't an option for that, maybe Nicolas can help out there.