Firstly: Making up Mustahabb Seclusion
It is mustahabb for the one performing voluntary seclusion to make up his time if he were to break it. This is the position of the Shafi`is, Hanbalis, and a position among Hanafis.
Secondly: Making up Votive Seclusion
Whoever vowed to seclude during some time, then he broke his decision or nullified it, then he must make it up. Consensus has been quoted on this by: Ibn Rushd, al-Zurqani, and Ibn Qasim
Thirdly: Making up Votive Seclusion on behalf of the Deceased
Scholars have differed over a guardian making up votive seclusion on behalf of one who vowed it then passed away before fulfilling it. This is according to two views.
The first: is that it is not mustahabb for the deceased's guardian to make it up on his behalf, though he may feed on his behalf if it was explicitly mentioned by him in a post-mortem directive. This is the position of the majority: Hanafis, Malikis, Shafi`is.
The second: is that it is mustahabb for his guardian to do so. This is the position of the Hanbalis, a position among Shafi`is, and the choice of Ibn `Uthaymin.