I need to lose about 5 dB from a 15 Watt exciter. Thanks.

Simple answer is no.

While you can build one with wirewound resistors , the normal formulars
will
not usually work at RF and you will have a lot of inductance to deal
with.
Even the socalled non-inductive resistors are only so at the audio
frequencies.

Can the inductive reactance be cancelled with capacitors? I'm mainly
concerned with 40, 30 and 20 meters.

The reactance will change with frequency. Do you want to be fiddling with
antenna tuners? There is a whole lot of stuff you need to know when
designing and building. A good place to start is with an ARRL Handbook.
They can often be found in public libraries, but they are an excellent
reference on the subjects you are dealing with. I own several because even
the older information is useful.