Sure!
So, on the hive, we have the model section, but there's designated section for HD models which you can find
here. To find this, along the top of the site you can go to Resources, and select the Models section - this area will display all the SD models you can filter through. Just above these model lists, there is "Reforged HD Models" and "Ported Models". One thing to be wary of with HD models is they take a lot more file space than the SD models, so your map can become quite big!
To import, within the World Editor, you will want to find the Asset Manager (previously referred to as Import Manager in older versions). From here, you can hit "Import" to find the files associated with whatever model you have downloaded. When doing this, be sure to include any relevant files associated with the import - some models use custom textures, and you will need to adjust the paths as specified by the model's creator. Many models use ingame textures, however, meaning you only need to import the model and you can specify whatever path you want. When importing a portrait model, make sure it has the same path as the model it's associated with, retaining the "_portrait" aspect of the import. For your purposes, the model imports themselves will not need their paths to be changed.