No, this isn't possible and shouldn't be done.
The reason the security slider was introduced was to allow granular but consistent choices, providing intervals between reduced attack surface and usability.
Setting a custom level would make you distinguishable from other Tor Browser users, which would defeat the anonymity provided by Tor Browser.
One of the considerations against the security slider is that each setting makes you look different from other Tor Browser users, breaking Tor Browser users up into a smaller anonymity set and making you more trackable. Tor Browser aims to put all users into one anonymity set.
If you only need some features sometimes and want to not have them at others, you should set the slider appropriately for each situation.