I am not entirely certain I understand what you are attempting to do, so I will give a few brief ideas and if these do not meet your specific question, please respond and elaborate.
The family interrelationship variables take there values from the PERNUM variable which identifies individuals within a household. There is some sample syntax available in the end notes of the Family Interrelationships page for attaching characteristics from one family member to another.
If you are trying to attach the characteristics of one person to the rest of the household (using household as a proxy measure for family, which may be problematic), you can create a data set containing only one person per household with the information you want to attach to the other household members (being sure to change variable names to prevent conflicts with the characteristics of other household members, for example, age becomes childs_age), and then merge one-to-many on serial.
If you are attempting to attach information not available in IPUMS-CPS that you downloaded from another data source, I recommend looking at thisresponse. If you are attempting to link persons across March samples, I recommend looking at thisresponse.
I hope this helps. Again, feel free to ask further questions.