Currently signature lines can only be added in Writer. It should be possible to add them in Calc too. Excel supports signature lines too.
https://support.office.com/en-us/article/add-or-remove-a-digital-signature-in-office-files-70d26dc9-be10-46f1-8efa-719c8b3f1a2d#__toc311526851
Samuel Mehrbrodt committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=9891fd076c30d353e9edfee9678f0b8e96d26238 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: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.