Currently signature lines can only be added in Writer.
It should be possible to add them in Calc too.
Excel supports signature lines too.
Samuel Mehrbrodt committed a patch related to this issue.
It has been pushed to "master":
tdf#117903 Add signature line feature to calc
It will be available in 6.2.0.
The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.