I'm working on designing a front-end data acquisition board for signal analysis, and have completed the schematic in LT SPICE. Now I want to import the schematic design into Altium Designer 18 in order to create the layout and gerber files for manufacturing, but I'm having trouble doing so. From that I've figured out so far, such things are normally done inside of the import wizard, but there's no option for LT SPICE files, and certainly none for .asc anyway, which is the file type LT SPICE outputs. This is especially confusing as Altium owns SPICE. Can anyone help me?
Just draw it again in Altium, with the real parts and database that will generate your BOM. There is no way to import much of value and it wouldn't save you much, if any, time anyway.
Then you will need to make sure you can find or create footprints for each part, and do the PCB layout in Altium.
Finally, you can generate the Gerber (and NC drill) files from the layout, from within Altium.
Nobody "owns SPICE". Spice stands for "Simulation Program with Integrated Circuit Emphasis" and is an open-source tool that is implemented by dozens upon dozens of different programs. Altium has its own built-in simulator based around SPICE but it does not "own SPICE". LTSpice and PSpice are other examples of programs that are built around the "SPICE" tool, but once again, Linear Technology and Cadence (the companies that make LTSpice and PSpice) do not "own" it.
To my knowledge Altium does not currently have an importer to pull in data from LTSpice.
If there is a way to run other spice netlists in altium, you could export a netlist from LT spice by using Tools->export netlist OR you can view the netlist in VIEW-> Spice Netlist
Altium doesn't own spice, it was a project started by UC Berkley. Many companies have their own version of spice.
Even if you do import a spice netlist into altium, you will still have to link up all of the models. It's going to be easier to redraw it in alitum.