XP existed for Itanium. Windows 2000 was the only "NT-based" version of Windows that was only ever publicly released on a single architecture.
PowerPC was supported from 3.51 to 4.0.
Alpha was supported from the very beginning to the late Windows 2000 betas. (Note that Windows on Alpha was only ever 32-bit. The Windows 2000 betas were supposed to add 64-bit support, but Alpha support was dropped before this happened.)
MIPS was supported from the beginning to 4.0.
Public Windows 2000 was only ever x86 32-bit. (There was a *VERY* late Windows Server 2000 Datacenter Edition for Itanium, but it was never "public" - it was given to a very select few companies that asked for it.)
Itanium support officially arrived with "Windows XP 64-bit Edition" - not "Windows XP Home Edition 64-bit" or "Windows XP Professional Edition 64-bit", the Itanium edition was its own separate "64-bit Edition" solely for the Intaium. It was a strange hodgepodge of features - most of the actual high-end business "Professional" features, while
missing some Pro features, and quite a few Home features.
That edition was *VERY* short-lived. HP was the only company that sold Itanium "workstations", and they only lasted about a year. Everyone saw that Itanium was only making actual sales in the server market, so MS discontinued XP for Itanium after a short time (it doesn't support later Itanium CPUs,) and concentrated on Windows Server 2003 support.
PowerPC support was limited to IBM/Motorola "PReP" compatible systems. Apple had been planning on making the Power Macintosh line PReP compatible, but it was determined to be too hard, so parts of the Macintosh firmware were added to PReP's replacement standard: CHRP. Apple was then planning on making Macs CHRP-compliant, but Steve Jobs came back, and both Mac clones and CHRP compliance went out the window.
Notably, one Mac clone maker was poised to release a PowerPC 750-equipped, CHRP-compliant Mac clone,
Motorola! But Steve Jobs... Motorola even got as far as to ship review units to magazines before they discontinued their Mac clone line. Mac OS 8 even has support for CHRP-based systems, but they had to be whitelisted by Apple. The only company to actually
ship CHRP-based systems was IBM, who only supported their flavor of UNIX (AIX) on it.
And Windows NT4.0 never supported CHRP, PowerPC support having been discontinued before CHRP systems shipped.
We were *VERY* close to having a Motorola CHRP system, though, that if it had shipped, might have convinced Microsoft to keep developing PowerPC support through Windows 2000 at least.
But, to get back to the question at hand - no, having the source code for Windows XP wouldn't be sufficient to just "target: PowerPC" in the compile engine. The oddball Itanium edition makes it clear that by XP, they had lost cross-platform compatibility, and it had to be added back for Windows Server 2003 (which later served as the basis for Windows XP for AMD64/x86-64 systems. Yes, that's right, Windows XP 64-bit on your Athlon 64 or late Pentium 4 was actually Windows Server 2003 with a Windows XP skin, rather than a direct compile of Windows XP. It even used the Server 2003 service pack numbering rather than the XP SP numbering.)
*POSSIBLE*, sure. But with a LOT of code wrangling.