Only the static ones.
For instance, in Objective C I frequently would add methods to NSString in a category (bundle of dynamically loaded methods) that made my life easier. ie
[myString asNetworkMessage] or whatever.
Smalltalk, ditto - adding asFoo methods is the accepted way of adding conversions.
Its only the pretend OO languages (C++, Java, C#) that have this annoyance.