I can't help with the Verizon MiFi 6620L, but their AC791L Jetpack (made by Netgear) has a pair of antenna ports.
As to the Sleek - be sure you have one designed for your carrier. Wilson went through a number of designs for them, including carrier specific models, one with a switch to go between carriers, etc. While the wrong one may boost some 3G connections, you need the correct model for 4G.
I have a Sleek with the switch, and it does provide a strong boost for Verizon (with the switch up, the position for the Verizon network). I've connected my WirEng Boat Antenna directly to the jetpack and had far less gain.
