hmmm... it looks like you can just combine several JFRs with simple

cat ./profile-* > /tmp/profile.jfr

and then generate a heatmap for all of the profile files:

jfrconv -o heatmap ./profile.jfr  /tmp/heatmap.html

at least it works and the result heatmap looks okay (it shows a graph for several hours), but I'm not sure if I didn't lose any data with this approach.