I have a dc to dc converter. The lowest setting is 3vdc. Can I run two 1.5v batteries in series and charge them?
Rechargeable 1.5 volt batteries are usually around 1.25 volts +/- so, yes, you can charge them from a 3 volt source.
Depending on the battery capacity, amount of discharge, and current capabilities of your DC/DC converter, you will probably need some type of charge controller circuit.
It may need to be a regulator circuit, a simple current limiting resistor, watching the charge rate with an inline current meter and voltmeter, or simply feeling the the battery temperature with your hand and knowing when to stop the charging.
There are many small battery chargers that use a 12 volt input, or even have a built-in solar cell to charge the batteries. Those would be an off the shelf solution that don't require any electrical engineering work.
Maybe something like that may work for you instead of trying to use a 3 volt supply?
Thank you for your help. Most of the questions I ask here will never be performed. I do spend some time and money on prepping. I am curious in case I do need it.