Adding equipment has to be done BEFORE you use it. If I want to add running shoes to a exercise what happened yesterday, and the shoe wasn't in the equipment list at that time is is impossible. Dating it back when adding is not possible.
I am running much longer than Garmin Connect exists, and I have some pairs of shoes that are not in the list.
And: If you want to give the shoes a realistic lifetime which is not 400 miles = 644 kms then you can't type it in, you have to pull on a wheel for some minutes.