DEFINE is a compile time directive so it can not be used to change stuff at runtime. I don't know how ADCIN works under the hood, ie if it's simply adding a PAUSUS for the duration of the sampletime or if it's actually setting acqusition time select bits of the ADC (which on the 18F4550 is in the ADCON2 register.)

If you need to change the acqusition time during runtime you're better off driving the ADC manually - which I believe you've received as an answer already.

Read chapter 21 in the datasheet a couple of times. It explains how to calculate the minimum conversion clock cycle time (TAD) as well as the minium acquisition time (TAcq). The acquisition time is derived from the TAD so you need to choose a TAD which allows for a long enough acqusition time.

/Henrik.