Dual-licensing OS.Cash and AGPL-3?
The underlying value underneath AGPL and OS.Cash are similar; both call out that something of value, something special, has been created and, if you use it, something is needed in return to keep things moving forward. In other words, an exchange of value is taking place, an agreement to support each other exists; this is not an unconditional give-away.
In the case of the AGPL, forever-access to the code is value provided in exchange for the software; such that, anyone and everyone can enjoy future improvements and derivatives. In the case of OS.Cash, wealthy organizations (specifically, Billion Dollar Businesses) need to pay for use of the software (in order that the developer is supported).
For the AGPL-3 projects that you already have, as long as you hold the copyright to all of the non permissively licensed code in the project, then yes, absolutely, you may dual-license under both the AGPL-3 and OS.Cash. Then you can give your users the choice of which license they use, allowing user's to select AGPL terms or OS.Cash terms. The risk here is that Billion Dollar Businesses may choose to use your software under the AGPL terms, for free, without purchasing an OS.Cash license for use.
Note that in section 10 of the AGPL-3 (and GPL-3), you are explicitly prohibited from charging any form of license fee, while the point of OS.Cash is that independently developed free-software should is not unconditionally free for Billion Dollar Businesses.