I've used Lightroom for several years and it has always bugged me that, under certain circumstances, the "add" option on the import dialogue is grayed out. As an example, let's say I'm shooting on location and writing files to a memory card in my camera. I bring a laptop to the shoot and I want to use Lightroom on the laptop only for image selection but not image processing. Image processing will be done later on a desktop machine. Ideally, I'd like to be able to insert the memory card into a reader and import into Lightroom using the "add" option but it's grayed out. Lightroom insists on copying the files to the laptop's local drive before doing anything with them. This can be time-consuming because all of the files need to be copied and there's really no benefit to copying them because they'll never be used on the laptop.
While I've accepted this limitation as working "as designed", I've run into again in a different way. I store my photo files on many drives and the workflow I've followed is to copy the files to the drive location and folder I want them then import using Lightroom with the "add" option. Recently, I added this cool little 512 GB USB3 external SSD drive that I plan on using as a working file drive. I plan on using this little SSD drive as a "current" area - when it fills up, I'll move the files to rotation drives to free up space on the SSD as working space. Well, unfortunately, the external SSD is seen by Lightroom like a card reader and the "add" option is grayed out. So, now I have to alter my workflow to accommodate the application. Are there any plans to make the "add" option available regardless of the source in the import dialogue?
I don't know why Lightroom won't allow you to use ADD when the photos are on a SSD ... however ...
No need to place photos on an SSD. You will not notice the speed difference. Thus, your workflow could be changed to an external hard drive without losing anything. See this study by Ian Lyons: Will an SSD Improve Adobe Lightroom Performance? | Computer Darkroom
I have exactly the setup you describe; my "Current" catalog and the photos it contains are on a usb 3 SSD drive and LR relates to it as a local drive and the Add option is operative. I believe your problem lies with the OS's defining of the drive as local or a removable storage device, since LR is only parroting info from the OS. In Windows that identification can be set in the Device Manager.