Question: Is there any way to charge the batteries in an Olympus DS3400 digital voice recorder?
No, unfortunately; you cannot charge the batteries as the DS3400 is not sold with rechargable batteries and does not have an AC port.  USB charging can only be performed on the Olympus DM420, DM520 & DS5000 digital voice recorders.
Verification Source: In-house testing
If  you have any additional questions, please do not hesitate to contact me.