I have some old humbucker can I use it on my classical acoustic guitar ?
I tried to use nylon strings. String didn't stay in tune and and sounded awful for me. Slid didn't sound on string at all. Nylon string was me like a rapist who at beginning is nice and soft and at the end rapes you and leaves you in mental pain. I hate almost all plastic sounds. Pure plastic sounds awful.
Without any plastic sounds excellent.
I use special steel strings made for classical acoustic guitar Thomastik-infeld Classic S-Series Flat Wound
I tried to make piezo pickup and It failed. And I didn't find the right jack for guitar. In online store they had picture of right jack . when it arrived it was wrong one.
I know that I can buy somewhere a awful pickup adapter 
I was thinking If I can use humbucker. I can make myself pickup adapter out of metal or wood etc. ?
For my cbg build I'm making magnetic pickup. Do I need to use potentiometer between amp and pickup?
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CB Gitty has jacks that have strap button, would still need to be installed from inside the guitar though and you may have to put it off center if the wood block in guitar is too thick for the threads. I think he has "push in" ones too, so you would only have to get the wire thru the hole and then can solder outside of the guitar and push pin/jack in.
I didn't know that there is easier way jack. Just push in and screw it on guitar body.
I'm going to use this If i can buy any. My problem is that I live in Estonia.
Thomastik strings haven't bent my neck. I used standard tuning only once or two and playing was hard
because action was high. In open g or d fingering is great. After 10 months strings sound great. Plectrum is bad. Breaks the wound round the core.
I know that jack works but I can't put guitar strap on it.
On display stand on store was right jack like there online store picture.
They sold me that wrong one. I tried to use same type jack again and failed. Getting jack to stay
on guitar body trough sound hole was very hard.
The Thomastik strings you chose have a steel core, so they should work with a humbucker. You can test it out by temporarily hooking the humbucker up to a jack, then plug it into an amp. Hold the humbucker close above the strings. Strum the strings and it should make sound.
The jack you're calling the wrong jack is similar to what I have used in the past. It should work (although disappointing that they sent you the wrong part.) I would let them know about the error.
It should be possible to make your own pickup adapter like you described.
You don't necessarily need a volume potentiometer, but it's nice to have one.