You cant insert a rubber oring as an insulator from the bottom even if you do take it apart. So honestly the steps ive listed as the only viable fix.
The top does come off, you just have to gently work it out with some pliers and rubber so as not to damage, and be careful cuz when it pops off, good chance youre going to break the leaks and have to re solder them.
So basically;
Yes you can take it apart, and no it is not hard, but you are likely to break the leads and have to resolder
No you cannot re insulate the center pin from inside, you have to tear or rip the old insulating ring out, and replace it with another insulatoer, as i suggested, a #60 o-ring works great