External Antenna connection on MFRC522
A quick review shows that the Lant has complimentary design (first sentence on P.81 of the data-sheet). This is also true for a coil antenna such as the one you found from Molex.

Short answer: Yes.
Not as short: You should verify antenna parameters to understand whether you need to tune the antenna to your reader. The typical application example shows there are components used to tuning so overall it seems possible.