Allan Domb has expanded his Rittenhouse Square holdings and picked up 1801 Walnut St., the building that houses Anthropologie. The acquisition adds to a growing portfolio of properties that the self-described condo king now owns in that part of Center City. In addition to having a stake in 1845 Walnut St., which is an office building, Domb within the last couple of years bought the Alison building at 1805 Walnut St. where Barnes & Noble is the anchor retail tenant as well as the space at 10 Rittenhouse…