This is an old thread but I couldn't find a response or documentation anywhere, so I wanted to point out that simply chaining the certificates in the UI now works.
i.e. in the certificate section, put the site certificate, then any chained certificates in the order of the chain going from site to root. Something like:
-----BEGIN CERTIFICATE-----
...
(base 64 encoded site certificate)
...
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
...
(base 64 encoded chain certificate)
...
-----END CERTIFICATE-----
etc