2 wires connected by a plastic wire-nut.  I want to further secure them by a liquid glue all inside the nut, that will harden permanent.?

Glue will reduce the quality of the electrical connection.

A wire nut, of the correct size for the wires, will make a connection that is mechanically and electrically sound, and hold the wires in compression.  It's hard to beat.

the option is solder, but no one does that but me 

if you staple the romex to the studs it will be secure if you nut it correctly 

what kind of stress do you worry about ?

Glue is an insulator.

Don't use it on an electrical connection!

Where is this wire being used?

What gauge is it?

Solid or stranded?

What voltage?

For   more security  just wrap the nuts  with    electrical tape.

The best for that is a tube of GE RTV, otherwise called, silicone rubber. It will seal the connection from moisture and not disturb the connection at all.

Secure them with a wire nut and then tape the connection up with electrical tape. It will not come apart and be secure. 

If you think there is any possibility of a wire nut coming loose, use a proper screw terminal junction box instead.

Or use crimped-on eyelet terminal tags, then a nut, bolt and anti-vibration washer to connect them together.

(Be sure to use a proper ratchet crimp tool in that case, not the nasty plier things that come with cheap terminal kits). 

Do that inside a plastic box such as this, replacing the small terminal block with a suitably large one or using it as a container for a bolted junction.

https://www.ebay.com/itm/3-Way-Electrical-Junction...

There is nothing you can safely add to a wire nut.

Ensure whatever you do complies with your local electrical regulations, if it's anything other than low voltage wiring.

Just leave it alone.  The wires are not going anywhere.

Liquid glue will interfere with the conductive properties of the joint. If you have the proper sized wire nut, you DO NOT need any further mechanical reinforcement.

you could solder them together and then put the wire nut back on