Depending on use, you can functionalize their surface using positively charged organic ligands, e.g. Quaternary amines end.

You could try polylysine. This will impart positive charge to alumina.

The literature shows that for alumina the calcination temperature affects the surface acidity/basicity in terms of -OH and -H groups. In suspension altering the pH alters the charge on the alumina surface.  You say nothing about your application (why do you need 'acidic alumina'?) or how the powder will be used.  There's no concept of zeta potential for a powder - it depends on the particle and its environment.  Are you confusing electrostatic charge with surface charge?  The former could be important if one is using some electrostatic application method to a surface.