Help to add missing rows to input

I'm quite new to Pentaho, so I need some guideness/advise.

My input is a CSV file like this (probably around 100.000 lines):
TAG1, datetime1, value1
TAG1, datetime2, value2
TAG1, datetime4, value4
TAG1, datetime5, value5
TAG1, datetime8, value8
TAG1, datetime9, value9

(there are some missing datetime's: 3, 6 & 7)

My output should be also a CSV like this:
TAG1, datetime1, value1
TAG1, datetime2, value2
TAG1, datetime3, value2
TAG1, datetime4, value4
TAG1, datetime5, value5
TAG1, datetime6, value5
TAG1, datetime7, value5
TAG1, datetime8, value8
TAG1, datetime9, value9

This output is going to be merged with another CSV and that combination is going to be written into a CSV file.

As you can see, a missing row is a copy of previous one with datetime increased. Gaps do not have the same size, so not always adding one row.

Can I do that with Pentaho? How? Maybe with Analytic Query? with Javascript?
For initial testing, a copy will be enough but new values could also be interpolated?

Thank you in advance.