Both the HLL (Algebird) and TDigest implementations we're using have a simple way to serialize a compressed representation. So basically just reading the row, merging the value currently stored, and writing the merged value back.
Depending on how many times you will write to the row, you could avoid having to do a merge on write by using APPEND_IF_FITS and just merging the byte arrays when you read.
It's nice that FDB gives you so much low level flexibility, you can do whatever you feel fits your use case.
