Kibana can with Scripted fields but that is not really converting the field , it is creating a new dynamic field
You need to actually send it to Elastic as a Int and Set the Mapping of your index so the field is a int not string
for example if you groking the field "%{NUMER:responseTime:int} adding the int will make it an integer (you can use float if it is a decimal) or you can do it with the "MUTATE" filter to convert the field from string to whatever
for mappings, consult the Mapping documentation - Good to define your data
https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping.html
you will need to re-index your data to change the mapping (or delete your data and send it in again)
https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-reindex.html
finally , and probably the most CPU intensive way
https://www.elastic.co/guide/en/kibana/current/scripted-fields.html
but I would recommend not doing that it will be slower and higher cpu.
I would Just make sure your data gets sent in to ELK in the right data format and have your mappings set appropriately.