I'm not sure exactly what your data looks like. How many lines are then to be replaced? Do the step numbers change and if they do, how? So i came up with a candidate data file that looks like this:

-----------step 1.
-----------step 1.
-----------step 1.
-----------step 2.
-----------step 1.
-----------step 1.
-----------step 1.
-----------step 2.
-----------step 2.
-----------step 2.
-----------step 3.
-----------step 3.
-----------step 2.
-----------step 2.
-----------step 3.
-----------step 3.

The following one liner will transform the data into hopefully something that is very close to want you want.

perl -pi -e's/(-----------step\s+)(\d+)\./$h{$2}||=$2;$h{$2}+=.1;$1.$h
+{$2}/ge' foo.txt; cat foo.txt
-----------step 1.1
-----------step 1.2
-----------step 1.3
-----------step 2.1
-----------step 1.4
-----------step 1.5
-----------step 1.6
-----------step 2.2
-----------step 2.3
-----------step 2.4
-----------step 3.1
-----------step 3.2
-----------step 2.5
-----------step 2.6
-----------step 3.3
-----------step 3.4
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)