Compiling in 32Bit mode, NewHexValue returns “BB2D06EBE” as expected,
but with One64 it returns “FFFFFFFFB2D06EBE”.
Compiling in 64Bit mode, NewHewValue returns “FFFFFFFFB2D06EBE” with 1
and One64, and “B2D06EBE” with One32.

Kem is right. This is doing the right thing. In 32 bit compiles the literal 1 is 32 bit. So adding 32 bit value (memoryblock.Int32Value) and a 32 bit value (1) yields a 32 bit value. That is converted to a 32 bit hex value. In a 64 bit compile the literal 1 is 64 bit. So adding 32 bit value (memoryblock.Int32Value) and a 64 bit value (1) yields a 64 bit value. That is converted to a 64 bit hex value and in order to make the memoryblock.int32value a proper 64 bit value it is “sign extended” . Since the high, or sign, bit of &hB2D06EBD is set to 1 that is replicated across the "upper bits to make the value a 64 bit signed integer. Then two 64 bit values are added and then hexed.

Many thanks to you, Kem (for the solution) and Jason (for explanation).
The solution works very well, 32 and 64 bits IDE or Compilation.

Jason, extending a 32bit hex value to 64bit, I would have expected filling additional bits with 0, but the higher for a sign value.
This would lead to a correct value of the 32Bit value, wheras replicating the higher bit of the 32Bit value leads to a wrong one (in 64Bit) if this bit is 1.
Is this a normal behavior or a bug in conversion between 32Bit Hex values and 64Bit Hex ones?

I believe I have to practice and test on such a problem in order to understand the mechanism…

This would be very bad and incorrect behaviour. When you fetch the Int64 value from something that is 32 bits sign extension has to happen otherwise things would unexpectedly change sign.
For instance, if you did:

dim i32 as int32 = -1
dim i64 as int64
i64 = i32

and did not do sign extension then i32 would be -1 and i64 would be some other positive value and that would be 100% wrong
The same holds true for your code (which could be written as):

Dim One64 As Int64=1
Dim MyMemoryBlock As New MemoryBlock(4)
dim intermediate as int64 = MyMemoryBlock.Int32Value(0)+One64
NewHexValue=Hex(intermediate)

the addition of Int32Value and One64 proceeds as:

find the common type that int32 value and one64 can be converted to to make the addition work

the common type is “Int64”

create a temporary that is of type int64

convert int32value to int64 and assign to the temporary from step2 ← this is where sign extension happens - so that the new int64 has the same value as the int32

now add the temporary and one64 giving a new int64 value temporary

assign the int64 to the resulting variable called “Intermediate”