Each skill by default comes pre-configured up to level 10 in the configs. This is mostly just to give an example of what PixelSkills can do, in terms of its core functionality and rewarding system. However, level 10 is not the max level that a skill can go. That's completely up to you, the user. Each skill has its own config file where its level up values are stored (in <skillName>LevelUps.conf), and it is in here where you can add levels, change how much EXP is needed to level up to those levels, etc etc. Since all skills' level up config files are formatted the same, just with changed values depending on the skill to help balance them a little bit, I'll only show one config file, specifically from the Breeder skill.
}By default, all players start on Level-1 in a skill, so there's no need to define how much EXP is needed to level up to Level-1 and so we start on Level-2. As set in this config, in the Breeder skill (which is where this config comes from) it takes 3 EXP to level up from Level-1 to Level-2. 7 EXP to level up from Level-2 to Level-3. Etc, etc.
To add more levels, like a Level-11, you just simply have to add it in the same format as the other levels here. A side note for you lazy typers out there, it is case sensitive and has to be "Level", not "level". :P