This is not a single external process, but three processes (top, egrep and tee), connected by a pipe.
Since this is already Unix specific, I guess you could just start a shell and pass your command line. The caveat is that it is not so easy to kill all the grandchildren processes, i.e. the children of the shell, top, egrep and tee.
Anyway, a naive solution is something like this (I cannot run your exact command, as it does not work on my system):
p <- processx::process$new("bash", c("-c", "seq 1 10 | sort"), stdout = "|")
p$read_all_output_lines()
#> [1] "1" "10" "2" "3" "4" "5" "6" "7" "8" "9"
You could use the pkill tool to kill the shell together with its children. (Note that pkill behaves wildly differently on different systems.) Although these particular processes should finish and quit quite quickly, anyway.