Those "sky textures" that come with FF: Can I replace or add to any of them with my own HDRI spherical sky maps?

FrameForge does not currently support HDRI but you can use your own spherical sky maps in FrameForge as long as they are in a format it understands, PNG, JPEG, etc.

You set Sky and Ground Textures by going to the SETS Menu and then either starting a new set or editing the current set's parameters as appropriate.  You'll see the following menu and you can change the Sky or Ground texture by selecting the appropriate Radio Box on the upper right of this dialog, then clicking  the [APPLY TEXTURE] button and selecting the one you want.

Not that there are several different ways to "map" a flat texture onto a curved surface, and FrameForge lets you choose the appropriate one for the sky image you selected from a drop-down under the central render of the Sky Dome.

Changing a set’s parameters is extreme­ly easy. Simply go to the SETS Menu and click EDIT SET PARAMETERS and you’ll see the following dialog where you can give it a name, define the default wall heights, their color, texture, door style etc. You can also change the base ground and sky textures and, if you are using the Pro or Stereo editions of FrameForge, you can also define this set’s geographical location which allows the program to automatically adjust the sun’s position based on the time of day.

In the center of the Edit Set Parameters dialog box you’ll see an image of a blue dome over a green square; these shapes represent your set’s sky and floor, respectively. To the right of this 3D image are two options, Sky Settings and Ground Settings and you should select the radio button of the one you want to change, and either remove the texture and choose a straight color, if desired, or click [APPLY TEXTURE] to select a new one from the many that FrameForge supplies or from your own image library.