I am currently updating my Sekai to modern components.
The original drivetrain was a Suntour Cyclone with RD cable above the chainstay and FD cable had a casing to it.
I would like to move the cables below the bottom bracket and it will require a cable guide under the bottom bracket.
The Sekai doesn't have any hole under the bottom bracket area, so drill a hole, glue it or just place and leave the cables to hold it?

To do this, you would also need to move the RD cable stop from the top to the bottom of the chainstay. If it's clamp on, no problem. Rotate it. If it's brazed on, I suggest you let it stay over BB routed. You would need to cut it off and braze a new one on the bottom, and then repaint.

Anyway, to install an under BB cable guide, yeah, you should drill and tap a hole.

And cable tension will hold it in place as long as it needs to be held in place.

RD cable stop is clamp on, so I will just flip upside down.
Thanks for inputs, I will go with shoegoo laying down in some drawer in the garage

ShoeGoo, or any other solvent-based adhesive, might mess with the paint (depending on the solvent, and its strength), in case that might bother you. Silicone sealant or acrylic bathtub caulk might be less likely to do that, though silicone that's not "neutral cure" type will off-gas acetic acid; don't know what that might do to paint.

Good point - will study before deciding on with glue to use. The paint is not in excellent condition, but don't want to put something really bad there.

nobody is going to notice a stray hole under the bb as long as it is done well.

Also known as a "drain hole."