InfluxDB data manipulation, manual insertion of data

I have a MQTT sensor reading rain data from my weather station and storing it in my InfluxDB database.
Unfortunately the data coming to Home Assistant needs to be multiplied by three in order to get an actual reading. I wrote a template sensor for that. Easy.
MQTT sensor has a forced_update parameter resulting in a nice row of data. Template sensor however updates only when the MQTT sensor value changes. So here I get data looking kinda like Troll’s teeth.
I have a battery of other sensors asking the database for rain values at various times. It seems to me that when there is is no data with a very close timestamp, these sensors are acting up.
So I searched around for a way how to force the template sensor to update regularly. Found and solved. Question remains what to do with all the data which is already in the database.
Is there a way (through Flux or any other means) to fill the gaps? It can be something along the lines of “find the value and create data points with that value once every minute until you find that the value changed, then take that value and repeat”. Or it could take every data point of MQTT sensor and create a template sensor datapoint with the same timestamp. Or any other way. I am more or less ok with semi-manual methods as well (download properly formatted CSV, change values and re-import).
I more or less know what I want to do but I have no idea how to make Flux do it for me. Or even whether I should try to do it with Flux or use different method. Any advice will be appreciated.