I should better explain my use case:
I would like to create my own seed by using dice throws. When I have a 99 digit base 6 number, I want to type that into Bitcoin Core on an offline machine/live os. Then I want to take the XPUB that is created, and transfer it to an online machine running Bitcoin Core as "watch only". Then I can create new receiving addresses on the online machine, which I will also use to create PSBTs.
I don't mind using a simple script or two if absolutely necessary, but I prefer to only use Bitcoin Core for security reasons.
Here's the thing: First you have to convert that 99 digit base 6 number into a format that Bitcoin Core understands since it can't just take any string. 99*6 is about 594 bits, divide that into 8 and it's slightly over 74 bytes, i.e. more than you'll need for the hdseed since as the others said, it is represented as a private key - those have 256 bits.
And maybe it's not the best idea to use the digits directly, after all, they have no special meaning besides entropy, but maybe you can use something like this method:
https://crypto.stackexchange.com/a/6177/945421 => 00
2 => 01
3 => 10
4 => 11
5 => 0
6 => 1
The numbers on the right are binary bits you add to the right of the sequence that you're making (just as you would with the dice). Then you can group sequences of 4 bits into hex characters once you have 256 bits.