Replace line range, defined by variable, with a string

I would like to replace a specific line range with Nan values. My line range is defined by variables for the first and last line I would like to replace, so for example:

startrow=3
endrow=6

Input file:

1
4
3
2
6
1
3
4

Desired output file:

1
4
Nan
Nan
Nan
Nan
3
4

I think this should be possible using sed. Help much appreciated!