For some phones it is possible but not available on the FP3 range in fact hardly any phones have a simple plug in to use option.
Here is a link to such a discussion and maybe Fairphone would consider the benefits of extra longevity for phones.
The following links are for those with a young heart and an old phone: Do not try any of these experiments unless you are prepared to fail, damage the phone, burn the house down and potentially set of an explosion that could decimate your environment, if you’re not doing that already.
I’m going to update this post with other links where people have succeeded in power other phones. I will make separate posts when there is some headway on what is possible for the FP3
Wow, I didn’t think it was a novel thought…
I have to say these are a bit extreme solutions, but the old cell phone is still in the closet, why not, I might try a good fire extinguisher nearby.
In any case, the cell phone is no longer just a mobility tool as mentioned in the second link. In many offices it is used, always from the same desk, to access the bank, generate pins, tokens and talk to employees who are around.
The use as a static device is to be considered as a feature: in the moment it is available, new uses could be figured.
~not currently with the default FP3 
~ though from the link in my first post some phones reportedly do.