Adobe Illustrator: Is it possible to add a custom stroke size (weight) into basic palette which starts with 0,25pt - 0,5pt - etc?
You cannot add your own value to the stroke palette manually. You would need a plugin for that. If you want to change the stroke weight of an existing object to a custom value that is not already offered, just select the object, and in the top bar type in the custom value in the Stroke numeric input.
Or open the Stroke window. and type in the value there.
As others have posted, you can't edit the increments in the Stroke Panel, but you can set the
Default paint style to start with a custom stroke weight.
You can create custom startup profiles that have strokes set to something specific. Essentially, you alter teh startup documents and set things how you want them... then save. So every time you open a new file with that document profile, you get the initial settings you want. While this doesn't make changing the stroke weight later any easier... it can let you start with a custom weight.
In fact, you don't even really need to edit the startup profiles....
You can create several default Graphic Styles that merely alter stroke weights to custom amounts. That would make editing stroke weights just a single click. Graphic Styles are meant to hold commonly used settings, that's their entire reason for having a panel.
Creating custom start up files: