I dont think you should have any problem at all importing a spice file in ADS. Just make sure your spice file is inside your project directory and that u have have checked the correct type of spice file in the import options. Ohter than that if u still have problems, u can do it manually for such a simple circuit.
I just tried myself to Import your circuit and works fine, here are the steps to see for yourself if u dont any mistake
1. Save your file as myname.sp (the extension of imported file should be sp)
2. Put your spice file at your project directory (if u dont it wont import anything)
3. Open a schematic window and go to FILE-->IMPORT, a new window with import options will open
4. On field "file type" u should have netlist, Press Browse go to your project directory and find your myname.sp file
5. Press "More Options" and make sure that u have, "Netlist Dialect" PSPICE, clicked First line is a comment, "Translated Output Format" ADS schematic. PRESS OK and OK again.
YOur file will be imported just fine !!
my ADS version is 2005A but i have done similar things is ealrier versions so u will be fine