I was wondering, do I really need to purchase some "boutique" eurorack module to do the 5v to line conversion.
And could I perhaps get some normal audio mixer and use some adapters on it?
-- soundmodel

Neither Ladik nor Doepfer are "boutique"; they are at the lower end as far as pricing.
I guess you could use a mixer with a -20 db pad on the input.

Know someone with some electronics knowledge?

-- wiggler55550

They are "boutique" in the sense that I don't see them as useful in other than eurorack modulars.
Using a general mixer would be more "generic". Allowing one to use "general purpose" hardware, rather than specialized.
I don't think the eurorack modules really do anything special, they just do the attenuating accurately, precisely to the needs of the hardware.