Custom modifications are always welcome and are priced on a case by case basis. Some of the more common requests include:

Install a DC jack or convert to standard 2.1mm "Boss" style barrel jack; typically $15+
Convert old ACA Boss pedals to run on standard power adapters
Install additional controls including gain, tone, volume, etc.
Add an expression pedal input to modulation and time-based effects
Convert pedals that require two 9V batteries to work with a single power source
Convert pedals to run at 18V from a single power source; usually for additional headroom in overdrives/boosts
Relocate jacks/switches for better placement on a pedalboard
Change LED color or brightness
"Dual Channel" mods to overdrive/distortion effects
Footswitchable "rate" knobs on modulation effects
Upgraded components for better audio quality

It would be impossible to list every options for every pedal but we list our most popular mods on our Standard Modifications page.