how can I make adresses of (parts) of buildings?

Don't split a single physical building up just for addressing purposes. There are a number of ways you can approach the problem:

• If the address of the building covers a range of street numbers, just record this, e.g. addr:housenumber=17-21
• If different addresses have separate physical entrances, add nodes for those using building=entrance and tag those with the house number.
• If buildings are physically joined but internally separate, such as terraced houses, use the terracing facility/plugin available on most editors to create the terrace. This will be easier and more accurate than doing it by hand.