They already do that; the WT Corp is known to, and has to publicly disclose, that it has an extensive investment portfolio encompassing many stocks in Wall Street. So yes, they invest in financial institutions.
Make no mistake about it. Money only flows one-way in JW-Land, from the rank-n-file Sheeple's wallets, to WatchTower's offshore bank accounts. No money ever flows BACK into the Congregations, unless absolutely necessary to repair/remodel a Kingdumb Hell for a quick sale.
The Congregations were pressured (AKA: scammed by WT HQ and CO's) to send all of their local Congregation savings to WT HQ a few years ago. They were told it was for "Safe keeping", and that if they needed anything, they could easily request their money back to do things. Think that's how it works? LOL!!!! They have to beg and plead to GET THEIR OWN FUNDS BACK, but 99% of the time WT HQ says, "REQUEST DENIED." Too bad, so sad! The WatchTower Org are total fraudulent crooks and charlatans! False prophets, for profits, hiding under the guise of a "charitable religion" here in the USA. Crooks!