I came across this interesting journal article called "The Ghost Battalion: Spaniards in the Waffen-SS, 1944-45." Check it out. Basically, what happened was, when Franco withdrew the Blue Division from the Leningrad front in 1943 under Allied pressure, some of them, convinced Nazis, stayed behind and joined the Waffen-SS. Other recruits came from Spaniards who had volunteered to work in Germany, Spanish Republican exiles held in prison camps, and from Spanish Nazis who crossed the border into France to volunteer. The author estimates there were at least one thousand of them. What's interesting about these guys is that they all joined super-Nazi organizations, either the Waffen-SS, the SD, or the Gestapo, and they joined up very late, many of them after the Normandy invasion made it clear to everyone that the Nazis were doomed. Many of them wound up in Leon Degrelle's Belgian SS battalion, and most of those guys got killed in the battle of Berlin. A lot of them served as anti-partisan troops in France and Yugoslavia, and a lot of them worked against the Anglo-Americans in France with the Gestapo; they didn't all serve on the Eastern Front, as the Blue Division did.
I wonder if this might not be the case with Enric Marco, the impostor who claimed to have been an inmate at Mauthausen and fooled everyone, to the point of being elected president of the association of Spanish ex-concentration camp inmates. Marco was forced to admit that he had actually volunteered to work in Germany, and he didn't get much more specific. Could he have been one of these guys, and his whole imposture an attempt at covering up his SS or Gestapo service?