While Linnworks does support adding a single value to an item specific, it does not support adding multiple values to an item specific.
This is supported directly on the eBay UI
It is also supported in both the new and old APIs provided by eBay
With eBay's continued push for Item Specifics, LW should support eBay's functionality to remain useful.
Its also worth noting, that if you do edit the listing through another mechanism or directly, LW will, I believe, overwrite that data when next updated. This means LW actively prevents you using it and a system which correctly / fully supports eBay (even if this is manually correcting things through the eBay UI)
1) Include a delimitator in Item Specific fields, that LW will convert into separately submitted entries - the quickest / easiest way to support this
2) Support multiple selections of item specifics within the Linnworks UI - the complete way to support this
3) Include the ability at configurator and template level to prevent LW updating certain parts of the eBay listing - e.g. exclude updating of item specifics. This is the complete way to allow other more appropriate tools to also manage listings without LW blindly overwriting data, allowing LW to be useful while allowing more useful tools to function.