Your SI_Write function looks quite like Windows Kernel32's WriteFile.

So, I would do this:

[DllImport("SiUSBXp.dll", SetLastError = true)]
static extern int SI_Open(uint dwDevice, ref IntPtr cyHandle);  

[DllImport("SiUSBXp.dll", SetLastError = true)]
static extern int SI_Write(IntPtr cyHandle, byte[] lpBuffer,
   uint dwBytesToWrite, out uint lpdwBytesWritten);

EDIT: I found this documentation USBXPRESS® PROGRAMMER’S GUIDE on the web, and it states that the SI_Write prototype looks actually much closer to WriteFile than I thought. The doc states this:

SI_STATUS SI_Write (HANDLE Handle, LPVOID Buffer, DWORD NumBytesToWrite,
DWORD *NumBytesWritten, OVERLAPPED* o = NULL)

It means the .NET prototype should be this instead:

[DllImport("SiUSBXp.dll")]
static extern int SI_Write(IntPtr Handle, byte[] Buffer,
   uint NumBytesToWrite, out uint NumBytesWritten, IntPtr o);

o is optional so you can pass IntPtr.Zero.