Can you make some explanation: what if F-value (float f)?

And m_mTransform is World matrix?

Naming failure ftw.

"float f" is meant to be "float z", therefore the 3rd coordinate, and therefore the depth for your text. m_mTransform initialially is an identity matrix, getting filled with the inverse (depending on how you sort your sprites - front to back or back to front, so you might want to try on that one) z coordinate. You can create it in the function call, but I decided to "cache" it in the class for (small) performance reason. So the better understandable function might go:

void D3DFont::DrawText(RECT rect, LPCWSTR lpText, DWORD dFlags, float z, D3DCOLOR color)
{
D3DXMATRIX mTransform;
D3DXMatrixIdentity(&mTransform);
m_mTransform(3,2) = 1.0f - z;
m_lpD3DSprite->SetTransform(&mTransform);
m_lpFont->DrawText(m_lpD3DSprite, lpText, -1, &rect, dFlags, color);
}

Nope, text is still at the top

sprite.Begin(SpriteFlags.AlphaBlend);
Matrix matr = Matrix.Identity;
matr.M32 = -5; //hardcoded, just a try variat, primitives min z is -1
sprite.Transform = matr; //transform sprite with this matrix
SignFont.DrawText(sprite, "o", new Rectangle((int)(screenPoint.X - 1), (int)(screenPoint.Y - 1),
(int)(screenPoint.X + 1), (int)(screenPoint.Y + 1)), FontDrawFlags.NoClip | FontDrawFlags.Center | FontDrawFlags.VerticalCenter,
new ColorBGRA(100, 0, 255, 255));
sprite.Transform = Matrix.Identity; // after rendering text cancel transform for other elements
.............................
sprite.End();