There is no expression function that can insert rows into an existing table. You can insert columns, for example here is a fairly complex table I've made that fetches some number of rows and then calculates the percentage of each row as it relates to the overall total.
It's using the var_set and var features available in 7.7 and higher:
filters
| var_set name="results" value={essql query='SELECT DestCityName, COUNT(*) as count from kibana_sample_data_flights group by DestCityName'}
| var name="results"
| var_set name="sum" value={ply expression={math "sum(count)"} | getCell "value"}
| var name="results"
| staticColumn name="sum" value={var name="sum"}
| mapColumn name="percent_of_sum" expression={math "count / sum" | formatnumber "0.0%"}
| sort by="count" reverse=true
The alternative to var_set is copy+pasting queries, but it's not as fast.