I don't usually rehydrate whole batches, but when I do run across one that does a little better with just a tad more moisture, I just pack the bowl and blow into the chamber out through the stem for a few puffs. Of course, Mrs Cosmic always makes a joke about me smoking my pipe wrong when she sees me doing this, ha ha.
But :::sigh::: if you are just bound and determined to rehydrate a whole batch, then you can set the tin or jar (open) inside a large plastic bin, alongside a bowl or mug of warm water, cover the bin, and wait for a day or two. Tobacco is an awesome absorber of ambient moisture. In fact, in Alabama, I don't have to do anything other than leave the tin open for a few days without the bin nor the mug of water. YMMV
Where are you located. Hasn't our mod, jpmcwjr suggested that you put your location in your profile to help us when you have questions like this?
I hope that helps.