Please have a look at page 20 of 116 in the waspmote technical guide. It clearly states that the battery can be charged via USB or a solar panel connected to the waspmote. There is even an LED indicator that shows the charging status.
I don't think the button cell battery is rechargeable. This battery is a disposable one like the ones you get on the PC motherboards. It lasts for many years as its purpose is only to keep the RTC running, which does not use too much power.
I hope you will find the above still useful.